Morphology and mechanical and viscoelastic properties of natural rubber and styrene butadiene rubber latex blends

The morphology and mechanical and viscoelastic properties of a series of bl ends of natural rubber (NR) and styrene butadiene rubber (SBR) latex blends were studied in the uncrosslinked and crosslinked state. The morphology of the NR/SBR blends was analyzed using a scanning electron microscope. The m orphology of the blends indicated a two phase structure in which SBR is dis persed as domains in the continuous NR matrix when its content is less than 50%. A cocontinuous morphology was obtained at a 50/50 NR/SBR ratio and ph ase inversion was seen beyond 50% SBR when NR formed the dispersed phase. T he mechanical properties of the blends were studied with special reference to the effect of the blend ratio, surface active agents, vulcanizing system , and time for prevulcanization. As the NR content and time of prevulcaniza tion increased, the mechanical properties such as the tensile strength, mod ulus, elongation at break, and hardness increased. This was due to the incr eased degree of crosslinking that leads to the strengthening of the 3-dimen sional network. In most cases the tear strength values increased as the pre vulcanization time increased. The mechanical data were compared with theore tical predictions. The effects of the blend ratio and prevulcanization on t he dynamic mechanical properties of the blends were investigated at differe nt temperatures and frequencies. All the blends showed two distinct glass-t ransition temperatures, indicating that the system is immiscible. It was al so found that the glass-transition temperatures of vulcanized blends are hi gher than those of unvulcanized blends. The time-temperature superposition and Cole-Cole analysis were made to understand the phase behavior of the bl ends. The tensile and tear fracture surfaces were examined by a scanning el ectron microscope to gain an insight into the failure mechanism. (C) 2000 J ohn Wiley & Sons, Inc.